820 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 77.2 



not distant future, an enterprising small-boat 

 designer ■will present a much more extensive 

 summary and digest of all available and useful 

 information, as N. L. Skene did some years ago 

 in his book "Elements of Yacht Design," and 

 as D. S. Simpson and P. G. Tomalin did more 

 recently [SNAME, 1951, pp. 554-611; 1953, pp. 

 590-634]. 



Manifestly, it is not possible to compress into 

 this chapter, with its illustrative examples, all 

 the essentials and fundamentals that have 

 appeared in each of several books on the design 

 of motorboats, Usted in the bibUography of Sec. 

 77.41, to say nothing of the valuable informa- 

 tion in the multitude of technical papers and 

 articles on this subject. It endeavors only to 

 cover certain hydrodynamic features of motorboat 

 design in a manner similar to the coverage of the 

 principal features of large-ship design in Chaps. 

 64 through 68. It stresses the differences in 

 characteristics and procedures necessitated by 

 the difference in size, as well as by the presence 

 of a major supporting force other than buoyancy 

 in the form of dynamic lift. 



As an illustration of the use of the data pre- 

 sented and the procedures described, there are 

 included in the chapter the preliminary hydro- 

 dynamic designs of two motorboats, to two 

 alternative specifications. One involves a fast 

 semi-planing hull and the other a high-speed full- 

 planing hull. 



77.2 General Considerations Relating to Mo- 

 torboat Design. For water craft of all kinds there 

 is little difference in behavior \vith size if care is 

 taken to maintain dynamic similarity of flow. 

 This is done when a self-propelled free-running 

 model, or a pilot model large enough to carry one 

 or more persons, is built as part of the develop- 

 ment work on a large project. Dynamic similarity 

 of flow is not fully realized if model tests include 

 towing of the model of a mechanically propelled 

 craft but exclude self-propulsion tests of that 

 model, when driven by a small-scale replica of 

 its propulsion device. This matter is discussed 

 further in Sec. 77.40. Since similarity of flow is 

 almost never completely achieved, as when 

 endeavoring to ruii simultaneously at the same 

 values of the Froude number and of the Reynolds 

 number for both model and ship, similarity is 

 maintained for that flow which is considered the 

 most important. This is the basis of the whole 

 technicjue of ship-model testing. 



Considering vessels of the displacement type. 



for which the speeds are usually restricted to T, 

 values less than 2.0 or 2.5, F„ < 0.60 or 0.74, 

 the comments as to water flow and ship behavior 

 of Chaps. 24, 25, and 26 of Part 2 are vahd 

 regardless of size, at least for all craft large 

 enough to carry an adult human. Except for 

 certain limitations on small craft because of the 

 accommodations which have to be provided for 

 human beings of nearly constant dimensions, the 

 design rules and considerations set forth in 

 Chaps. 66, 67, and 68 of Part 4 apply to small 

 craft as well as to large ones. W. F. Durand 

 pointed out as long ago as 1907, in his book 

 "Motor Boats; A Thoroughly Scientific Discussion 

 of Their Design, Construction, and Operation" 

 [Int. Mar. Eng'g., London and New York, 1907, 

 Library of Congress number YM341.D9] that: 



". . . the selection of principal dimensions, hull coeffi- 

 cients and parameters, underwater form, and the size 

 and form of appendages is handled in almost exactly the 

 same way as for a large vessel." 



The principal difference — and unfortunately also 

 a principal difficulty — hes in estimating the total 

 weight of the finished small craft, especially if 

 it is of some new type. There is another difference, 

 discussed in Part 6 of Volume III under Wave- 

 going, in that the short wind and ship waves to 

 be met by a motorboat or motor cruiser are 

 considerably steeper than the long waves having 

 the same ratio of wave length to ship length for a 

 large vessel. 



The motorboat has wavegoing adventures, even 

 in waters that are considered sheltered. Velox 

 waves from passing vessels of larger size or greater 

 speed are steep and sometimes troublesome, as 

 are the waves stirred up in shallow areas by 

 sudden squalls and storms. As a rule, the slowing 

 down of motorboats in shallow and restricted 

 waters is taken for granted, so that no special 

 hull shapes are required for these conditions. 



Motorboats of the semi-planing type represent 

 a considerably more difficult design problem than 

 those of the displacement type. The change of 

 trim when underway becomes a major design 

 parameter and the necessity for an accurate 

 estimate of the weight becomes much more acute. 

 Moreover, there is a rather wide variety of 

 possible hull shapes from which the proper choice 

 has to be made. It must be admitted that in the 

 past these problems have not had the benefit of 

 the systematic empirical and practical study, to 

 say nothing of the scientific and analytic research 



